1. Field of the Invention
The invention generally relates to WLAN (Wireless Local Area Network) transmitters and corresponding integrated circuit chips and methods, and in particular to WLAN transmission techniques where data is transmitted in two or more different transmission modes.
2. Description of the Related Art
A wireless local area network is a flexible data communications system implemented as an extension to or as an alternative for, a wired LAN. Using radio frequency or infrared technology, wireless LANs transmit and receive data over the air, minimizing the need for wired connections. Thus, wireless LANs combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: FHSS (Frequency Hopping Spread Spectrum) and DSSS (Direct Sequence Spread Spectrum) systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum, is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible as far as it relates to direct sequence spread spectrum technology, but it adopts a new modulation technique called CCK (Complementary Code Keying) which allows the speed increase.
The CCK modulation can generally be described as a modification of MOK (M-ary Orthogonal Keying) modulation using codes of complex symbol structure.
The CCK technology allows for multi-channel operation and employs the same chip rate and spectrum shape as the 802.11 Barker code spread functions.
CCK can be considered as a form of M-ary code word modulation where one of M unique signal codewords is chose for transmission.
Further extensions to the IEEE 802.11 standard exist. For instance, the IEEE 802.11a and 802.11g specifications use the OFDM (Orthogonal Frequency Division Multiplexing) technique which is a wireless transmission technique that splits signals into sub signals that are then transmitted at different frequencies simultaneously. The 802.11g version of ODFM uses a combination of BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), and QAM (Quadrature Amplitude Modulation), depending on the chosen data rate.
Thus, a variety of different modulation types and methods exist within 802.11 compliant WLAN systems. Not all of the possible transmission modes have to be supported by each individual WLAN device, but multi-mode WLAN devices exist that support at least part of the possible modes.
An example of a conventional WLAN transceiver device is the Am1772 wireless LAN chip set which is depicted in FIG. 1. As apparent from the figure, the device comprises a baseband/MAC (Medium Access Control) unit 100 which includes a baseband section 110 and an MAC section 115. Both sections are connected via a baseband/MAC interface unit 120 which is media independent.
The MAC section 115 comprises an input/output bus host interface which is connected via an I/O bus to an external I/O bus host interface 180. The input/output bus host interface of the MAC section 115 is further connected to a frame composer 145 and a timer 150.
The baseband section 110 comprises baseband inner and outer receiver units 125, 130 and a baseband transmitter unit 135 to perform baseband data processing in both directions. Baseband data processing refers to signal processing after having shifted the frequency from the radio frequency domain in the reception path, and before doing the shift in the transmission path. The baseband section 110 further comprises a control logic 140 for controlling the baseband receiver and transmitter units 125, 130, 135 and the baseband/MAC interface unit 120.
The WLAN transceiver device of FIG. 1 further comprises an RF (Radio Frequency) transceiver 105 that is connected to the baseband/MAC unit 100 to interchange data which is received or which is to be transmitted. As the interchanged data is digital data, the RF transceiver 105 comprises digital-to-analog converters 165, 170 in the transmission path and analog-to-digital converters 155, 160 in the reception path. The reception path further comprises an LNA (Low Noise Amplifier) and an AGC (Automatic Gain Control) unit for selectively adjusting the reception gain. Further, there is a VCO (Voltage Controlled Oscillator) unit which is connected to a PLL (Phase Locked Loop) unit.
As apparent from FIG. 1, the WLAN transceiver device further comprises a power amplifier 185 which receives an analog output signal to be transmitted, from the RF transceiver 105. The power amplifier 185 is controlled by the control logic 140 of the baseband section 110 in the baseband/MAC unit 100 via a power amplifier control signal. The control logic 140 further provides a transmitter/receiver switch signal to switch operation of the device between a reception mode and a transmission mode. Further, the control logic 140 provides an antenna switch signal for selecting one of two (or more) antennae 190.
When operating a WLAN transmitter device such as that of FIG. 1, a number of factors need to be taken into account to achieve optimum transmission conditions. For instance, the output power with which data is transmitted is a crucial point. Evidently, when increasing the output transmission power, the signal-to-noise ratio at the WLAN receivers that receive the transmitted signals is increased. However, another crucial point is the signal quality which should be as high as possible. As there are non-linearities in the power amplifier 185, the signal quality decreases when the output power is increased since there may occur severe signal distortions. This may lead to a situation where the signal-to-noise ratio is low but signal demodulation and decoding nevertheless leads to higher error rates at the receiver because of the reduced signal quality.
It is therefore often found difficult to decide on the compromise between signal-to-noise ratio and signal quality. This may lead to situations where a WLAN receiver cannot successfully demodulate and decode a signal either because of too much noise (i.e. too weak data signals) or because of distorted signals.